1. Field of Invention
This invention relates to a device for measuring fatigue strength and fatigue damage of metallic and composite structures and to a method for predicting the service life remaining on those metallic and composite structures to which the measuring device is attached.
2. Description of the Prior Art
Structural and design engineers must be able to determine the fatigue strength and fatigue life of any material that is used or being considered for use as a load-carrying component and which is or will be subjected to a repetitive or cyclic stress loading condition. This requirement stems from the fact that repetitive stress on a structure will eventually cause a material failure in that structure due to fatigue in the material comprising the structure being tested. Further, as a load-carrying structure is subjected to repetitive cyclic loads, structural and design engineers must be able to monitor the effects of these load conditions to determine the remaining service life of the structural member so as to take it out of service before failure occurs.
Engineers have extensively studied the fatigue life of structural materials to more accurately determine the current state of fatigue damage and to more accurately predict the remaining service life in these structures. These studies have shown that fatigue strength is a function of the material comprising the mechanical or metal structure being tested or measured, the manner in which that material has been treated, the ambient temperature in which the structure exists or operates, the amount of stress applied to the structure, and the number of stress cycles the test member undergoes. These studies have also shown that such structures are subject to fatigue failure when they are subjected to repetitive stresses that are lower in magnitude than the ultimate stress of the materials making up the structures being tested. Further, these studies have shown that the service life of a given structural material is inversely proportional to the applied stress; i.e., the greater the applied stress, the shorter the service life of the structural material.
To determine the current state of fatigue damage and/or predict the remaining service life in these structures, test engineers have typically relied upon any number of fatigue monitoring devices such as the fatigue detectors, fuses, gauges, indicators, monitors, predictors, sensors, testers, and transducers taught by the prior art. These conventional devices are typically attached to the structure being monitored so that the test elements are aligned with the direction of the maximum principal stress applied to the structure being tested. As such, these conventional fatigue measuring and/or monitoring devices were capable of monitoring fatigue damage in only one fixed direction. Further, these earlier fatigue gauges typically contained only one test element which necessitated multiple tests on the structure being tested or, alternatively, the attachment of multiple gauges to obtain the desired values of fatigue damage or service life remaining.
Those that contained multiple test elements such as U.S. Pat. No. 3,572,091 issued to McFarland (1971), U.S. Pat. No. 5,319,982 issued to Creager (1994) and U.S. Pat. No. 5,425,272 also issued to Creager (1995) were limited by the costly or time-consuming requirements that the test elements be of the same material as the structure being tested or that the test elements be cracked, notched, or otherwise structurally weakened to ensure that the test element experienced material failure before the structure being tested. Similarly, U.S. Pat. No. 3,786,679 issued to Crites (1974), U.S. Pat. No. 4,639,997 issued to Brull (1987), or 5,355,734 issued to Kajino (1994) were limited by the requirement(s) that the test elements be of the same material as the structure being tested or that the test elements be “cracked,” “notched,” or otherwise structurally weakened to ensure that the test element experienced material failure before the structure being tested.
Those which embodied multidirectional monitoring or indicating devices such as the device taught by U.S. Pat. No. 6,443,018 B1 issued to Lee et al.(2002) were limited in that they were designed to only measure or monitor structures with different lengths of artificial cracks or structures with a “weak point” such as a welded joint. Other gauges containing multiple test elements such as that taught by U.S. Pat. No. 4,081,993 issued to Leonhardt et al. (1978) were limited in that they were designed to measure compressive stress only.
Structural and design engineers had concerns in the laboratory as well. The relationship between applied stress and service life of any given structural material or component is typically shown by plotting the applied stress (“S”) on a structural material or component against the number of cyclic applications (“N”) required to induce failure at that particular stress level. In order to obtain the S-N curve of any new material or composite material for which failure data is not available, test engineers have typically fabricated that material or composite into a test element as suggested by the American Society of Testing Materials (“ASTM”) handbook. These test engineers would then subject those specimens to cyclic loads of constant magnitude until the specimen failed. This process would then be repeated upon another specimen which would be subjected to repetitive cyclic loads of induced stress of a different magnitude until failure occurred. Test engineers would repeat these tests until they were able to obtain an acceptable number of data points to determine that material's performance under a number of load conditions. These repetitive tests were costly and time-consuming.